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Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 159–171www.elsevier.com/locate/jastp

The response of the high-latitude ionosphere to IMF variations

J.M. Ruohoniemi ∗, S.G. Shepherd, R.A. GreenwaldApplied Physics Laboratory, The Johns Hopkins University, 11100 John Hopkins Road, Laurel, MD 20723-6099, USA

Received 1 December 2000; accepted 6 April 2001

Abstract

The convection of plasma in the high-latitude ionosphere is strongly a2ected by the interplanetary magnetic 3eld (IMF)carried by the solar wind. From numerous statistical studies, it is known that the plasma circulation conforms to patterns thatare characteristic of particular IMF states. Following a change in the IMF, the convection responds by recon3guring into apattern that is more consistent with the new IMF. Some early studies reported that the convection 3rst begins to change nearnoon while on the dawn and dusk 5anks and on the nightside it remains relatively una2ected for tens of minutes. Work byRidley et al. (J. Geophys. Res. 103 (1998) 4023–4039) and Ruohoniemi and Greenwald (Geophys. Res. Lett. 25 (1998)2913–2916) that was based on measurements with more global sets of instruments challenged this view. A debate ensued as tothe true nature of the convection response. We follow the arguments of Lockwood and Cowley (J. Geophys. Res. 104 (1999)4387–4391) and Ridley et al. (J. Geophys. Res. 104 (1999) 4393–4396) by reviewing recent results on the timing of theonset of the convection response to the changed IMF. We discuss the timing problem from the perspectives of observationsand modeling. In our view, the onset of the ionospheric response to changed IMF is globally simultaneous on time scales ofa few minutes. A physical basis for the rapid communication of e2ects in the dayside convection to the nightside has beendemonstrated in magnetohydrodynamic simulations. We also o2er some cautionary notes on the timing of convection changesand the use of global assimilative techniques to study local behavior. c© 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Convection; Ionosphere; Solar wind; Magnetosphere interactions

1. Introduction

The plasma of the earth’s ionosphere at high latitudes isusually observed to be in motion. Because of the incompress-ibility of the plasma, a displacement or acceleration withina localized region will generally give rise to compensatorydrifts over a much larger area. A number of statistical studieshave shown that the global circulation of plasma conformsto patterns that are characteristic of the interplanetary mag-netic 3eld (IMF) carried by the solar wind. Indeed, the mostrecent statistical models essentially prescribe the pattern ofconvection based solely on the magnitude and orientationof the IMF (e.g., Rich and Hairston, 1994; Weimer, 1995;Ruohoniemi and Greenwald, 1996).

∗ Corresponding author. Fax: +1-240-228-6670.E-mail address: [email protected]

(J.M. Ruohoniemi).

The preeminence of the IMF factor is most easily ex-plained by the dominant role played by magnetic reconnec-tion on the dayside magnetopause (Cowley, 1982). Wherethe magnetic 3eld imposed by the solar wind encountersthe geomagnetic 3eld in a favorable orientation, energy andmomentum may be transferred across the magnetopause(Dungey, 1961). For a southward IMF, reconnection mayproceed at points equatorward of the high-latitude cusp lead-ing to an antisunward 5ow of plasma into the polar cap. Theplasma eventually returns to the dayside along the dawn anddusk 5anks of the auroral zone resulting in a two-cell pat-tern of plasma circulation. Tubes of geomagnetic 5ux are‘opened’ into the solar wind at the dayside reconnection siteand returned to dipolar form at a reconnection site in themagnetotail. The azimuthal component of the IMF stronglyin5uences the reconnection geometry and hence the orien-tation of the dayside 5ows and the shape of the overall con-vection pattern (Cowley et al., 1991).

1364-6826/02/$ – see front matter c© 2002 Elsevier Science Ltd. All rights reserved.PII: S 1364 -6826(01)00081 -5

160 J.M. Ruohoniemi et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 159–171

The newer statistical models cited above largely agree onthe basic IMF dependencies of high-latitude convection. Thepatterns they generate are remarkably similar given the verydi2erent sources of measurements, namely, ion drift mea-surements from DMSP satellites (Rich and Hairston, 1994),electric 3eld measurements from the DE 2 satellite (Weimer,1995), and backscatter from ground-based HF radar(Ruohoniemi and Greenwald, 1996). It could be said thata consensus has emerged in the community regarding thestatistical characterization of the pattern for stable IMF con-ditions. The statistical patterns certainly cannot reproducethe variability in the convection described by Codrescu etal. (1995) and the parameterization by IMF must be in-adequate when the other causes of convection (substormreconnection, viscous interaction, neutral wind coupling)are comparable in their e2ects. Nonetheless, a paradigm hasbeen established for the basic dependence of the convectionon the IMF for quasi-static conditions.

It has been more diHcult to agree on the manner in whichthe high-latitude convection pattern recon3gures in responseto a change in the IMF. In part, this can be attributed to the

Fig. 1. MLT dependence of the ionospheric response time deduced by cross-correlating an event of 5uctuating Bz with CANOPUS groundmagnetometer data (after Saunders et al., 1992).

diHculties in making observations on spatial and temporalscales suHcient to resolve the evolving pattern. Only withinthe last 10 years, the observational and analytical techniqueshave progressed to the point where the instantaneous patterncan be mapped on a global scale. Certain key questions cannow be addressed. These include:

• Is the onset of the convection response to changed IMFlocalized or global?

• What are the time scales for the pattern to recon3gure?• Is the onset of a response conditioned by the existence of

thresholds?• To what extent is variability in the convection not due to

variation in the IMF?

Important progress on all of these points can be expectedwithin the next 5 years.

In this review, our focus is on the 3rst question as this hasgenerated the most controversy to date. We encourage thereader to become familiar with the exchange of views con-tained in the comment by Lockwood and Cowley (1999)and the reply by Ridley et al. (1999). In the next section,

J.M. Ruohoniemi et al. / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 159–171 161

Fig. 2. The Cowley–Lockwood model of the excitation and expan-sion of 5ows in the dayside ionosphere following the commence-ment of magnetic reconnection at the dayside magnetopause (afterSaunders et al., 1992).

we will brie5y review the background of the controversy.Then we summarize recent developments in observationsand modeling studies. We believe that this research narrowssome of the disagreement.

2. Background

The 3rst attempt to time the propagation of a convectionresponse was made by Lockwood et al. (1986). They ob-served e2ects in the ionosphere that followed a southwardturning of the IMF with the EISCAT radar. They con-cluded that the response propagated away from noon witha phase velocity of 2:6 km=s. This corresponds to a delayon the dusk (midnight) meridian relative to noon of about15 min (30 min) at 72◦ invariant latitude. (Here and in thefollowing, for ease of comparison, we characterize all re-sults on propagation in terms of time delay). This work wasfollowed by others that found comparable delays (Etemadiet al., 1988; Todd et al., 1988; Saunders et al., 1992). Fig. 1shows a plot of the local time dependence of the responsetime from Saunders et al. (1992). These authors correlateda case of 5uctuations in the z-component of the IMF withde5ections in magnetograms collected with the CANOPUSmagnetometers. Relative to the time of earliest onset (9–11MLT), the delays approaching the dawn and dusk meridi-

ans climb to values greater than 10 min. These results wereinterpreted in terms of the Cowley and Lockwood (1992)model of the excitation of plasma 5ows for changed IMFconditions. As shown for a southward IMF turning in Fig. 2,the Cowley–Lockwood model posits an initial perturbationthat is con3ned to the area of the ionospheric footprint ofthe cusp. This is followed by a gradual expansion to earlierand later MLTs as more open 5ux is added to the polar cap.

Fig. 3. Analysis of the recon3guration of the convection patternfor a northward turning of the IMF as performed by Ridley etal. (1998). The upper plot shows the base potential pattern beforethe convection change. The subsequent series of plots shows con-tours of residual electric potential obtained by subtracting the basepotential pattern from the patterns realized during the period ofrecon3guration.


Fig. 4. Sequence of maps of the line-of-sight velocities measured by the SuperDARN HF radars during the southward turning of the IMFrecorded on November 24, 1996 and discussed by Ruohoniemi and Greenwald (1998). The sources of the measurements are indicated by theletters identifying the approximate locations of the radars (T : Saskatoon, K : Kapuskasing, G: Goose Bay, W : Stokkseyri, E: Pykkkvibaer,F : Finland).

The time scales for the expansion are taken to be consistentwith the observations of tens of minutes of delay from noonto the nightside.

This picture was challenged by Ridley et al. (1998), whobuilt on the earlier work of Ridley et al. (1997). Theseresearchers used the AMIE technique applied to magnetome-ter data to map patterns of electrostatic potential through pe-

riods of changing IMF. This approach had previously beenused by Knipp et al. (1993) to study the recon3guration ofthe pattern during a northward turning of the IMF. The par-ticular innovation introduced by Ridley et al. (1997, 1998)was the use of residual potential patterns whereby an ini-tial pattern is subtracted from subsequent patterns to high-light the manner in which the recon3guration progresses. An


example is shown in Fig. 3 for a south-to-north turning ofthe IMF. The initial perturbation (seen in the map for 08:27UT) is clearly global. This is seemingly inconsistent withthe Cowley–Lockwood depiction of the response onset forwhich the initial perturbation is con3ned to a small area onthe dayside. Ridley et al. (1998) concluded that their re-sults implied that the onset of change in the convection pat-tern following an IMF turning occurs simultaneously on allMLTs on time scales of less than about 1 min (or perhaps2 min, allowing for the sampling rate of the magnetome-ters). Ridley et al. (1998) do not challenge the validityof the Cowley–Lockwood model on smaller time scalesthan this.

Ruohoniemi and Greenwald (1998) presented a case studyof the response of convection to a sudden southward turn-ing of the IMF. The velocity data measured directly by theHF radars of the SuperDARN network (Greenwald et al.,1995) were examined for evidence of the onset of a responseover the MLT sector stretching from noon to almost mid-night. This approach avoids complications that result fromthe inversion of magnetometer data for electric 3eld and theapplication of global assimilation techniques (see below).Ruohoniemi and Greenwald (1998) described an onset thatwas very nearly simultaneous on the 2-min resolution ofthe SuperDARN observations. Fig. 4 shows a sequence ofmaps of the line-of-light velocity obtained from the radarsfor a time interval that includes the onset of new IMF ef-fects in the convection. The data are plotted in the magneticcoordinate system described by Baker and Wing (1989).For each velocity determination, an arrow is drawn that ex-tends from a dot at the point of measurement in the directionof the 5ow. The arrow is both color-coded and scaled inlength in accordance with the velocity magnitude. The stepbetween plots is 4 min in order to show the evolution fromnorthward to southward IMF conditions. Noon MLT is to-ward the top of each plot. The changes in the velocitiesnear noon were very dramatic, with the 5ows reversingfrom sunward (characteristic of strong Bz+) to antisun-ward. The velocities also changed in the other MLT sec-tors, generally becoming larger in magnitude. These authorsconcluded that the pattern recon3gured very quickly andglobally following the arrival of the southward IMF turn-ing. These results were more consistent with the 3ndingsof Ridley et al. (1998) concerning the nature of the initialresponse.

The interpretation of the results of Ridley et al. (1998)was argued in the exchange between Lockwood andCowley (1999) and Ridley et al. (1999). Lockwood andCowley (1999) showed that a day-to-night progressioncould be discerned in the plots of residual potential bytracking the individual contours. Ridley et al. (1999) ac-cepted this but responded that this motion did not invalidatetheir conclusion that the initial e2ects were globally simul-taneous and hence still inconsistent with the earlier reportsof MLT dependence in the time of onset. We remark thatthe day-to-night progression cited by Lockwood and Cow-

ley (1999) is obviously consistent with a faster rate ofrecon3guration on the dayside. This tendency is apparentin the plots of Fig. 4 and does not con5ict with the fact ofa global onset.

The controversy then hinges on whether there is a signif-icant simultaneous response in the convection at MLTs thatare distant from those directly a2ected by the changed con-ditions of dayside reconnection. This issue can be addressedwith observations, or, more indirectly, by appealing to mag-netohydrodynamic (MHD) models or through considerationof theoretical concepts. In the next section, we discuss someof the complications encountered in addressing this issueusing measurements and global assimilation techniques.Then we discuss the recent 3ndings from observational andmodeling studies.

3. Some complications

It is worth reviewing some of the diHculties that are en-countered in studying the timing of changes in the convec-tion that follow IMF changes. Some concerns have alreadybeen expressed in the literature (e.g., Lockwood and Cow-ley, 1999). Here we develop some di2erent concerns thatwe feel should be recognized.

Timing issues: In most studies of the response of con-vection to IMF variations, an e2ort is made to estimate thetime of arrival of the IMF change at the magnetopause.This typically involves estimating the orientation of IMF‘phase fronts’ in the solar wind and the positions of thebow shock and magnetopause and some modeling of thepropagation within the magnetosheath. Several approachesare possible and, in general, these render results that varyconsiderably (e.g., Ridley et al., 1998). From the presumedarrival at the magnetopause some additional time, usu-ally 1–2 min, is then applied to account for propagationinto the high-latitude ionosphere. Confusion sometimesarises in discussions of response times from the severalchoices that can serve as the reference point for timingthe delays:

(i) the moment of presumed arrival of the new IMF at themagnetopause,

(ii) the moment of presumed arrival of the new IMF in thehigh-latitude ionosphere,

(iii) the moment of the 3rst response of the high-latitudeionosphere to the change in IMF.

The third choice, the one that we have preferred, is feasibleonly when a large portion of the high-latitude ionosphereis under observation. If the speci3c interest is the man-ner in which the ionosphere recon3gures, the details of thepropagation of the new IMF are largely irrelevant. In fact,it can be misleading if errors in the modeling of the IMFpropagation contribute to the determination of delays in theionosphere.


We have then to agree on when a response is apparent inthe convection that can be reasonably attributed to a changein the IMF observed at the position of an upstream satel-lite monitor. Several diHculties arise in this regard. First ofall, variability in the IMF and structure in the solar windcast doubt on whether every variation seen by the satellitein fact reaches the magnetopause (e.g., Collier et al., 1998).Less well appreciated is the impact of variability in the iono-sphere on the identi3cation of a response. Even for reason-ably stable IMF conditions, we 3nd that the point-to-pointand sample-to-sample variability in the ionospheric convec-tion velocity is pronounced, as 3rst discussed by Codrescuet al. (1995). The cause of this variability is unknown. Asa practical matter, it complicates the task of identifying un-ambiguously the onset of a response to an IMF change. Wemight expect that very marked changes in the IMF, such assudden north-to-south turnings, would stand out clearly inthe observational records. Indeed, as shown by Ruohoniemiand Greenwald (1998), the main e2ect of such a change islarge-scale and dramatic, but even here the possibility thatmore localized perturbations precede the large response can-not be ruled out.

Shortcomings of potential maps: In their comment, Lock-wood and Cowley (1999) proposed an alternative interpre-tation of the potential maps that had been generated byRidley et al. (1998). We have some more basic concerns.One is the use of global assimilative techniques to character-ize local behavior. Procedures such as AMIE (Richmond andKamide, 1988) and the APL potential 3tter (Ruohoniemi andBaker, 1998) ingest distributed measurements and output aglobal pattern that is a ‘best-3t’ in the sense of reproduc-ing as nearly as possible the individual measurements whilepreserving global behavior that is physical and reasonable.An example of a physical constraint is the requirement thatthe 3tted velocity 3eld be consistent with a potential electric3eld, E=−grad�, even if the individual measurements arenot. An example of a ‘reasonableness’ constraint is the use ofstatistical model data to 3ll in areas where measurements arelacking. The essential point is this: in a global optimizationprocedure, there is a tendency to ‘globalize’ local behavior.Consider the sudden appearance of a localized high veloc-ity feature in a subset of the available measurements. If thecoverage of the surrounding area is sparse, the solution forthe global potential pattern will likely adjust itself to accom-modate the need for greater in5ow to and out5ow from thehigh-velocity area. The degree to which this occurs dependson certain weightings and tradeo2s that a2ect the balancebetween reproduction of local behavior and reasonablenessof the overall map. The cautionary note that we sound here isthat the globalizing tendency of the 3tting procedures com-pels us to check our results for consistency with the localmeasurements.

A related problem arises in the interpretation of the mapsof residual potential as de3ned Ridley et al. (1998). A 3-nite residual potential results from di2erencing global quan-tities, namely, the potential distributions obtained by 3tting

measurements from two periods. It is possible to have asigni3cant residual potential emerge over an area with nochange in the local measurements. One then has to ques-tion whether a convection response has really occurred. Thatthis can come about is easily seen: the 3tting proceduremay faithfully reproduce the potential gradients associatedwith the (unchanged) local convection while assigning newpotential values to the local contours because of changesrecorded elsewhere. One then obtains a 3nite residual po-tential and infers the onset of a local response that was notapparent in any of the local measurements. Di2erences inthe characterization of the convection response may some-times arise between the global and local approaches fromthis cause.

4. Recent work: observations and modeling

In this section, we review recent experimental and model-ing results that bear on the question of how the high-latitudeionosphere begins to respond to the arrival of changed IMF.Some of this material was presented and discussed at the S5Symposium of the 2000 S-RAMP meeting held at Sapporo,Japan. We conclude with our view of how matters currentlystand.

Findings on the response of convection to IMF changeswere recently presented by Khan and Cowley (1999). Theseauthors argue that there is a signi3cant MLT dependencein the time of response onset. They performed a correla-tion analysis on IMF data from the IMP-8 satellite andconvection velocity measurements from the EISCAT facil-ity. A large number of individual events were consolidatedinto a database for reduction by statistical methods. Fig. 5summarizes their results. Here, the response time is reck-oned from the presumed time of arrival of the IMF changeat the magnetopause. For each MLT hour, the smaller ofthe response times determined separately for the meridionaland zonal components of the convection velocity is plot-ted. A best-3t line is overlaid as a solid trace. It is inter-esting to note that the minimum delay is found at 14 MLTrather than at noon. Although the scatter is considerable,the distribution is consistent with somewhat longer delayaway from this meridian. The maximum response delay isabout 6 min, calculated roughly as 8 min (0 MLT) −2 min(14 MLT). That is, on average, the nightside begins to re-spond within 6 min of the 3rst sign of a response on thedayside. This is a longer delay than the 1 min claimed byRidley et al. (1998) but not very di2erent from the 2–4 mincited by Ruohoniemi and Greenwald (1998). Thus the dis-agreement regarding the degree of simultaneity is reducedto minutes.

Lu et al. (2001), following the example of Ruohoniemiet al. (1998) used a stackplot presentation to analyzethe response of the southward IMF turning of November24, 1996 in magnetometer data. Their result is shown inFig. 6. Four meridional chains of magnetometers provided


Fig. 5. MLT dependence of the onset of the convection response to IMF change as determined from cross-correlation analysis of EISCATand IMP8 data (after Khan and Cowley, 1999).

Fig. 6. Stackplots showing the horizontal magnetic perturbations that followed the southward IMF turning of November 24, 1996. Thecoverage provided by the chains of magnetometers extended from near noon to midnight MLT (after Lu et al., 2001).


Fig. 7. Stackplot showing the e2ect of a southward IMF turn-ing recorded by the Geotail satellite on equivalent 5ows deducedfrom ground-based magnetograms. The upper panel shows the IMFz-component at Geotail time delayed by 10 min. The next fourpanels show the changes in the equivalent 5ows in the direction ofmaximum 5ow change at each station. The approximate MLT ofeach station is indicated. The time shifting of the IMF data alignsthe southward turning with the onset of signi3cant e2ects in theequivalent 5ows. The vertical dashed lines at 5-min spacing high-light the increase in recon3guration time away from noon (fromMurr and Hughes, 2001).

observations that extended from near noon to midnightMLT. In all sectors, strong e2ects were recorded withina few minutes of 2110 UT. This agrees with the con-clusion of Ruohoniemi and Greenwald (1998) on thenear-simultaneity of the IMF response measured on timescales of a few minutes. This is also the most completepresentation to date of the e2ects of an IMF turning on mag-netometer data and tends to con3rm the 3ndings of Ridleyet al. (1998).

Magnetometer data have also been analyzed by the groupat Boston University lead by J. Hughes. Their results havebeen presented at several venues by D. Murr, including theS5 Symposium of the 2000 S-RAMP meeting. Althoughcon3ned mainly to the dayside by conductivity considera-tions, their results also show nearly simultaneous responseonsets at all MLTs. Fig. 7 shows an example of e2ects inmagnetograms that followed the passage of a southward IMFturning at the Geotail satellite. The onset of recon3gurationwas observed nearly simultaneously across the 12–21 MLT

sector. The total spread in the times of response onset wasno more than 1–2 min. It was pointed out by D. Murr at theS-RAMP meeting that while the onsets occur at virtually thesame time for all the magnetometers, the maximum pertur-bations do not. Rather, the magnetometers at MLTs awayfrom noon tend to change more slowly. In the example ofFig. 7, the recon3guration clearly proceeded most rapidlynear noon. If one were to cross-correlate the magnetograms,the best correlations with the noon data would be obtainedfor somewhat later times on the dusk 5ank. The lengthen-ing of the recon3guration. time away from noon might helpexplain the MLT dependence of the onset time reportedin some of the correlation studies, such as that of Saunderset al. (1992).

The e2ects of strong IMF turnings are readily apparentin the observations carried out with the SuperDARN HFradars (Ruohoniemi et al., 2001). The onsets are generallyglobal on the 2-min time scale of the radar observations.Fig. 8 presents an example of the arrival of a south-ward IMF turning. The maps span the time of responseonset in the ionosphere. The map for 1312–1314 UT showsdramatic e2ects at all the MLTs under observation. Theseinclude an increase in the number of velocity estimates,which is due to an increase in the amount and intensity ofHF backscattering. The backscatter is also seen to extend tolower latitudes at all MLTs, consistent with an expansionof the auroral zone. Where velocities can be compared ina before-and-after sense, the magnitudes have generallyincreased from the 1304–1306 UT. The perturbations inthe velocity show de3nitively the onset of a response in thehigh-latitude convection. We see that ionospheric responsesare apparent both in the velocities and in the general condi-tions of HF backscattering and that the onsets are globallysimultaneous.

We can combine time series of the various IMF and iono-spheric measureables to examine the timing problem moreclosely. Fig. 9 shows a stackplot that illustrates the magni-tude of the change in the ionosphere that follows the arrivalof the southward IMF turning. From this, we infer that ane2ective delay of 34 min from observations of the IMF atthe position of the Wind satellite to signi3cant e2ects inthe ionosphere. We may carry this analysis further to theindividual MLT sectors. Fig. 10 shows the results for thenumber of velocity estimates and the maximum line-of-sightvelocity. Within each 3-h MLT sector, a dramatic changewas recorded in one or the other parameter within a fewminutes of 1310 UT. We conclude from consideration ofnumerous examples of this kind that the onset of dra-matic e2ects in the high-latitude ionosphere following IMFchanges is globally simultaneous on time scales of a fewminutes.

The new results have stimulated several e2orts to un-derstand the timing of the convection response from theperspective of modeling. Shepherd et al. (1999) analyzedthe propagation of IMF phase fronts from the bow shockto the magnetopause using data from upstream satellites


Fig. 8. An example of the e2ects of the arrival of a southward IMF turning in the observations of the SuperDARN HF radars. The 3rstclear response was seen in the 2-min scan beginning at 1310 UT.

while Geotail was positioned near the subsolar magne-topause. Fig. 11 shows an example. The propagation withinthe magnetosheath was based on the gas dynamic modelof Spreiter and Stahara (1980). Because of the variationin propagation speed with distance o2 the sun–earth line,new IMF comes into contact with a large portion of themagnetopause nearly simultaneously. The draping that

results greatly reduces the time needed to communicate in-formation about the new IMF throughout the high-latitudeionosphere.

A study has been performed with the MHD global sim-ulation code of Fedder et al. (1995) to help elucidate thephysics of the convection response. Slinker et al. (2000)modeled the southward IMF turning of November 24, 1996.


Fig. 9. Stackplot of data from the Wind satellite and the SuperDARN HF radars for an extended interval on October 2, 1998. The framesshow (i) the position of the Wind satellite in GSE coordinates, (ii) the IMF z- and y-components delayed by 34 min, (iii) the number ofvelocity measurements available from the radars within the grid cells de3ned by Ruohoniemi and Baker (1998), (iv) the largest line-of-sightvelocity magnitude seen by any of the radars, and (v) the latitude of the equatorward boundary of the HF backscatter as de3ned by Shepherdand Ruohoniemi (2000). The delay applied to the IMF data aligns the southward IMF turning with the onset of dramatic e2ects at 1310 UTin all three parameters (vertical dotted line).

Fig. 10. Stackplots of SuperDARN data for the October 2, 1998 interval. The 3rst plot repeats the time series of counts and maximumvelocity magnitude from Fig. 8; the remaining plots show the behavior of these parameters within distinct 3-h MLT intervals. Dramatice2ects were seen in all sectors within 2 min of the nominal onset time of 1310 UT.

Some of their results are shown in Fig. 12. Simulated con-vection patterns are plotted for a sequence of times that spanthe time of arrival of the IMF turning in the high-latitude

ionosphere. The 3gure also shows plasma convectionvelocities projected into the dusk quadrant of the mag-netic equatorial plane and velocities perpendicular to the


Fig. 11. Draping of IMF 3eld lines over the dayside magnetopause as modeled by Shepherd et al. (1999). The thicker lines show theintersection of the, X –YGSM plane with planes containing the IMF at four distinct times (1551; 1652; 1701; and 1713 UT). When the 3rste2ects of the IMF transition were observed in the ionosphere at 1703 UT, 3eld lines of the new IMF were draped over most of the daysidemagnetopause.

magnetic 3eld in the noon-meridian plane. There is goodconsistency between the patterns and those derived by 3t-ting the SuperDARN line-of-sight velocity measurements.In particular, the convection through the preceding intervalof northward IMF was characterized by reverse two-cellconvection on the dayside with sunward 5ows on the noonmeridian. Following the arrival of the southward turning, theconvection quickly recon3gured into the more conventionalglobal two-cell pattern with strong antisunward 5ows in thenoon sector. Slinker et al. (2000) found that perturbationsin the convection on the nightside followed the onset ofperturbations on the dayside by less than 5 min. Physically,this could be understood in terms of the propagation of ararefaction wave on closed 3eld lines with 5ow toward thesubsolar region needed to supply the magnetic 5ux to thereconnection site. The simulation also reproduced the sud-denness and simultaneity of the velocity changes observedwith the SuperDARN radars by Ruohoniemi and Green-wald (1998). Thus, reasonable agreement has been foundbetween observations of the global response and MHDmodeling results.

5. Conclusion

In our view, the onset of e2ects in the ionospheric con-vection at high latitudes due to changed IMF is globallysimultaneous on time scales of a few minutes. Numerousobservations by radars and magnetometers support this con-clusion. Taking a somewhat contrary view, Khan and Cow-ley (1999) argued that there is a discernible spread in onsettimes but found that it is typically only about 6 min. Fol-lowing southward IMF turnings, in particular, the Super-DARN radars detect dramatic changes in the intensity andextent of HF backscattering that are consistent with a global

ionospheric response. The global aspect of the response isreproduced in MHD simulations.

One caveat is in order. Workers in the 3eld have naturallyfocussed on the large e2ects in their data sets that followIMF changes. These have been found to have global onsets.The possibility that smaller, more localized perturbationsprecede the onset of the global transformation cannot beruled out.

There remains the task of reconciling this conclusionwith the earlier 3ndings. One suggestion, made by D.Murr, allows that correlation analyses of perturbation am-plitudes distributed in MLT might not properly associatethe perturbation onset times. There might also be some sen-sitivity to local e2ects, as would occur when observationsrestricted to a certain latitude failed to detect the onset ofa change in an MLT sector until an expansion of aurorale2ects brought a change within view. This sensitivity wasin fact discussed by Todd et al. (1988) and Etemadi et al.(1988) but perhaps was not fully resolved. When the exactnature of the evolution of the high-latitude ionosphere isdetermined with the aid of the more comprehensive obser-vational networks that are now operating, it may be possibleto simulate the earlier results and explain the di2erences.

Acknowledgements

We gratefully acknowledge our discussions with theparticipants of the S5 Symposium of the S-Ramp meetingheld at Sapporo, Japan. This work was supported by NSFgrant ATM-9819891 and by NASA grants NAG5-8361and NAG5-8037. Operation of the SuperDARN radarsin the northern hemisphere is supported by the nationalfunding agencies of the US, Canada, UK, France, andJapan.


Fig. 12. MHD simulation of the e2ects of the arrival of the southward IMF turning of November 24, 1996. The 3rst column shows maps ofthe global potential pattern and includes 5ow vectors where the velocity is greater than 100 m=s. The second column shows 5ow vectors in theequatorial plane where velocity magnitude is greater than 30 km=s. The third column shows the component of the 5ow velocity perpendicularto the magnetic 3eld in the noon–midnight meridional plane where the magnitude is greater than 30 km=s (after Slinker et al., 2000).

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